•The Moncuni (Lanzo Massif, W-Alps) ophiolite experienced subduction to 600C∘ and 2.1 GPa at dry conditions.•Ductile flow at temperatures >700C∘ prior to subduction.•Seismic fracturing at ...eclogite-facies conditions recorded by pseudotachylyte.•Preservation of pre-subduction assemblages and pseudotachylyte glass during subduction.•The Moncuni ophiolite is the proxy for the rheology of subducting dry oceanic slab.
The ophiolitic peridotite and gabbro of Moncuni (Southern Lanzo Massif, Western Alps) retain pre-subduction mantle-to-oceanic, high-temperature (>700C∘) ductile fabrics. These fabrics are overprinted by seismic fracturing and faulting associated with pseudotachylytes. Within the gabbro, the pseudotachylytes preserve dry glass and pristine microlites. The occurrence of rare, minute garnet and the static development of eclogite-facies assemblages in local hydrated domains indicate that pseudotachylytes experienced subduction conditions of 600C∘ and 2.1 GPa. The exceptional survival of glass and the absence of post-oceanic ductile deformation demonstrate prevailing dry conditions during the entire Alpine subduction and exhumation path. Dry conditions inhibited reaction kinetics and viscous flow. In contrast, the majority of the Alpine ophiolites, derived from the upper hydrated portions of the oceanic lithosphere, show pervasive fluid-assisted metamorphism and ductile deformation. The Moncuni body can, therefore, be regarded as representative for the rheological behaviour during subduction of seismic, dry, deeper oceanic lithosphere that is rarely exhumed to the Earth's surface. In Moncuni, the brittle-ductile transition of dry oceanic rocks is constrained to be between 600 and 750C∘. This temperature range corresponds to the observed cut-off of intermediate-depth seismicity within subducting slabs. We infer that the base of the seismic layer corresponds to the brittle-ductile transition of a dry slab rather than the locus of antigorite breakdown triggering earthquakes by dehydration embrittlement.
We report an experimental and microstructural study of the frictional properties of simulated fault gouges prepared from natural limestone (96 % CaCO
3
) and pure calcite. Our experiments consisted ...of direct shear tests performed, under dry and wet conditions, at an effective normal stress of 50 MPa, at 18–150 °C and sliding velocities of 0.1–10 μm/s. Wet experiments used a pore water pressure of 10 MPa. Wet gouges typically showed a lower steady-state frictional strength (
μ
= 0.6) than dry gouges (
μ
= 0.7–0.8), particularly in the case of the pure calcite samples. All runs showed a transition from stable velocity strengthening to (potentially) unstable velocity weakening slip above 80–100 °C. All recovered samples showed patchy, mirror-like surfaces marking boundary shear planes. Optical study of sections cut normal to the shear plane and parallel to the shear direction showed both boundary and inclined shear bands, characterized by extreme grain comminution and a crystallographic preferred orientation. Cross-sections of boundary shears, cut normal to the shear direction using focused ion beam—SEM, from pure calcite gouges sheared at 18 and 150 °C, revealed dense arrays of rounded, ~0.3 μm-sized particles in the shear band core. Transmission electron microscopy showed that these particles consist of 5–20 nm sized calcite nanocrystals. All samples showed evidence for cataclasis and crystal plasticity. Comparing our results with previous models for gouge friction, we suggest that frictional behaviour was controlled by competition between crystal plastic and granular flow processes active in the shear bands, with water facilitating pressure solution, subcritical cracking and intergranular lubrication. Our data have important implications for the depth of the seismogenic zone in tectonically active limestone terrains. Contrary to recent claims, our data also demonstrate that nanocrystalline mirror-like slip surfaces in calcite(-rich) faults are not necessarily indicative of seismic slip rates.
809 deep IODP Hole U1473A at Atlantis Bank, SWIR, is 2.2 km from 1,508‐m Hole 735B and 1.4 from 158‐m Hole 1105A. With mapping, it provides the first 3‐D view of the upper levels of a 660‐km2 lower ...crustal batholith. It is laterally and vertically zoned, representing a complex interplay of cyclic intrusion, and ongoing deformation, with kilometer‐scale upward and lateral migration of interstial melt. Transform wall dives over the gabbro‐peridotite contact found only evolved gabbro intruded directly into the mantle near the transform. There was no high‐level melt lens, rather the gabbros crystallized at depth, and then emplaced into the zone of diking by diapiric rise of a crystal mush followed by crystal‐plastic deformation and faulting. The residues to mass balance the crust to a parent melt composition lie at depth below the center of the massif—likely near the crust‐mantle boundary. Thus, basalts erupted to the seafloor from >1,550 mbsf. By contrast, the Mid‐Atlantic Ridge lower crust drilled at 23°N and at Atlantis Massif experienced little high‐temperature deformation and limited late‐stage melt transport. They contain primitive cumulates and represent direct intrusion, storage, and crystallization of parental MORB in thinner crust below the dike‐gabbro transition. The strong asymmetric spreading of the SWIR to the south was due to fault capture, with the northern rift valley wall faults cutoff by a detachment fault that extended across most of the zone of intrusion. This caused rapid migration of the plate boundary to the north, while the large majority of the lower crust to spread south unroofing Atlantis Bank and uplifting it into the rift mountains.
Key Points
No evidence of a high‐level melt lens with gabbros intruded at depth, then emplaced to high levels by crystal mush diapirism, plastic deformation, and faulting
Diving over the crust‐mantle boundary on the transform wall found no primitive cumulates, and evolved gabbro was intruded directly into the mantle at the transform
Primitive cumulates needed to mass balance MORB must lie at depth near the crust‐mantle boundary, while the few diabase dikes encountered intrude the gabbro
Smectite clays are the main constituent of slipping zones found in subduction zone faults at shallow depth (e.g., <1‐km depth in the Japan Trench) and in the decollements of large landslides (e.g., ...1963 landslide, Vajont, Italy). Therefore, deformation processes in smectite clays may control the mechanical behavior from slow creep to fast accelerations and slip during earthquakes and landslides. Here, we use (1) laboratory experiments to investigate the mechanical behavior of partly water‐saturated smectite‐rich gouges sheared from subseismic to seismic slip rates V and (2) nanoscale microscopy to study the gouge fabric. At all slip rates, deformation localizes in volumes of the gouge layer that contain a “nanofoliation” consisting of anastomosing smectite crystals. “Seismic” nanofoliations produced at V = 0.01, 0.1, and 1.3 m/s are similar to “subseismic” nanofoliations obtained at V = 10−5 m/s. This similarity suggests that frictional slip along water‐lubricated smectite grain boundaries and basal planes may occur from subseismic to seismic slip rates in natural smectite‐rich faults. Thus, if water is available along smectite grain boundaries and basal planes, nanofoliations can develop from slow to fast slip rates. Still, when nanofoliations are found highly localized in a volume, they can be diagnostic of slip that occurred at rates equal or larger than 0.01 m/s. In such a case, they could be markers of past seismic events when found in natural fault rocks.
Key Points
Nanofoliations develop at subseismic to seismic slip rates in partly water saturated smectite fault gouges
Nanofoliations may deform by frictional slip in smectite water‐lubricated basal planes and grain boundaries
Strain localized nanofoliations can be diagnostic of past slip events at rates larger than 0.01 m/s in natural fault rocks
Recurring subsolidus re-equilibration of granitic feldspars induced by fluid infiltration events provides a record of fluid–rock interactions that affect large volumes of the Earth's continental ...crust. This has a direct bearing on the interpretation of the present-day granitic rock mineralogy and geochemistry. We examine Palaeoproterozoic grey and red-stained granitoids from the Simpevarp and Laxemar areas in SE Sweden, particularly focusing on consecutive feldspar replacement reactions, to provide an in-depth understanding of subsolidus re-equilibration of granitic rocks with hydrothermal fluids. The apparently most unaltered grey granitoids contain highly porous oligoclase grains that enclose crystallographically continuous microcline relicts. This texture suggests that the oligoclase is already secondary and may be a replacement product of original microcline. Oligoclase is progressively replaced by albite (∼An9) along polysynthetic twinning and intragranular fractures. The features of this replacement are characteristic of a dissolution–reprecipitation mechanism. Fine-grained mica (sericite) is closely associated with the albite porosity within micron-sized pores observable with scanning electron microscopy as well as in nanopores imaged with transmission electron microscopy. The reddening phenomenon in the vicinity of fractures is contemporaneously related to the K-feldspathization of sericite, which is restricted to the altered oligoclase. Submicron size hematite precipitation within orthoclase pores at the replacement front results in the red coloration. The complex associations between the fluid–feldspar reactions indicate that the replacement reactions may be due to sequential fluid infiltration events and that the granitoids have undergone extensive subsolidus re-equilibration, changing the original magmatic mineralogy. Therefore, the effects of large-scale re-equilibrations of granitic rocks through hydrothermal convection systems should be more closely considered.
Serpentinization-fueled systems in the cool, hydrated forearc mantle of subduction zones may provide an environment that supports deep chemolithoautotrophic life. Here, we examine serpentinite clasts ...expelled from mud volcanoes above the Izu–Bonin–Mariana subduction zone forearc (Pacific Ocean) that contain complex organic matter and nanosized Ni–Fe alloys. Using time-of-flight secondary ion mass spectrometry and Raman spectroscopy, we determined that the organic matter consists of a mixture of aliphatic and aromatic compounds and functional groups such as amides. Although an abiotic or subduction slab-derived fluid origin cannot be excluded, the similarities between the molecular signatures identified in the clasts and those of bacteria-derived biopolymers from other serpentinizing systems hint at the possibility of deep microbial life within the forearc. To test this hypothesis, we coupled the currently known temperature limit for life, 122 °C, with a heat conduction model that predicts a potential depth limit for life within the forearc at ∼10,000 m below the seafloor. This is deeper than the 122 °C isotherm in known oceanic serpentinizing regions and an order of magnitude deeper than the downhole temperature at the serpentinized Atlantis Massif oceanic core complex, Mid-Atlantic Ridge. We suggest that the organic-rich serpentinites may be indicators for microbial life deep within or below the mud volcano. Thus, the hydrated forearc mantle may represent one of Earth’s largest hidden microbial ecosystems. These types of protected ecosystems may have allowed the deep biosphere to thrive, despite violent phases during Earth’s history such as the late heavy bombardment and global mass extinctions.
The occurrence of the quartz‐carbonate alteration assemblage (listvenite) in ophiolites indicates that ultramafic rock represents an effective sink for dissolved CO2. However, the majority of earlier ...studies of ultramafic rock carbonation had to rely on the surface exposure of reaction textures and field relationships. Here we present the first observations on ultramafic rock alteration obtained from the 300 m deep BT1B drill hole, ICDP Oman Drilling Project, allowing for a continuous and high‐resolution investigation. Hole BT1B recovered continuous drill core intersecting surface alluvium, 200 m of altered ultramafic rock comprising mainly listvenite and minor serpentinite bands at 90 and 180 m depth, and 100 m of the underlying metamorphic sole. Textural evidence suggests that the carbonation of fully serpentinized harzburgite commenced by non‐equilibrium growth of spheroidal carbonate characterized by sectorial zoning resulting from radially oriented low‐angle boundaries. In the serpentinite, carbonate spheroids are composed of alternating magnesite cores and dolomite rims, whereas texturally similar carbonate in the listvenite is composed of Fe‐rich magnesite cores and Ca‐Fe‐rich magnesite rims. The distinct compositions and mineral inclusions indicate that the carbonation extent was controlled by fluid accessibility resulting in the simultaneous formation of limited carbonate in the serpentinite bands and complete carbonation in the listvenite parts of BT1B. The presence of euhedral magnesite overgrowing spheroidal carbonate in the listvenite suggests near‐equilibrium conditions during the final stage of carbonation. The carbonate clumped isotope thermometry constrains carbonate crystallization between 50 °C and 250 °C, implying repeated infiltration of reactive fluids during ophiolite uplift and cooling.
Key Points
Large‐scale carbonation resulted in pervasive replacement of serpentinite by the magnesite‐quartz‐fuchsite (listvenite) assemblage
Clumped isotope thermometry indicates carbonation temperatures between 50 °C and 250 °C
Carbonation textures indicate a transition from non‐equilibrium spheroidal growth to near‐equilibrium euhedral growth
Serpentinization environments are key locations that support microbial communities by the abiogenic formation of reduced species associated with peridotite alteration. Here we studied partially ...serpentinized peridotites from the Chimaera seeps (Turkey), an active continental serpentinization system that vents highly methane‐rich fluids, to investigate the impact of water‐rock interaction on the sulfide and metal mineralogy and its implications on supporting microbial communities. Using high‐resolution scanning electron microscopy, electron microprobe analysis, and transmission electron microscopy we found diverse pentlandite decomposition features with precipitation of secondary sulfides including millerite, heazlewoodite, as well as Cu‐bearing sulfides, native Cu, and awaruite (Ni3Fe). Awaruite forms dense veinlets to single crystal platelets tens of nanometers in size, which is formed by desulphurization of pentlandite. In addition, the nanometer‐sized awaruite platelets are intimately intergrown with serpentine suggesting its growth during peridotite alteration by a dissolution‐precipitation process, likely associated with the interaction of methane‐ and H2‐rich but highly sulfur‐undersaturated fluids. Based on sulfur isotope signatures we infer a mantle and mid‐ocean ridge origin of the sulfide minerals associated with the first stage of partial serpentinization and awaruite formation. Subsequent and ongoing continental fluid‐rock interaction causes significant sulfide decomposition resulting in the formation of porosity and the release of, amongst others, H2S and Fe. These species may likely provide a source of nutrients for active microbial communities in these comparatively nutrient‐starved, low‐temperature continental serpentinization environments.
Key Points
High disequilibrium conditions induce skeletal awaruite growth during continental serpentinization
Pentlandite dissolution creates fluid pathways and nutrients for microbial life
Sulfur isotope compositions document ocean floor and subsequent continental serpentinization processes
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